Why we use the present simple for itineraries?
Solution 1:
Your information is incomplete. The present simple is often used for intended actions in the future, as long as the time is made clear in some other way. It is used in this way particularly as part of a list of intentions or engagements.
So in conversation, on its own,
I'm going to Manchester next week.
is more common, but
I go to Manchester next week.
is perfectly good, especially as part of an itinerary. (eg "... and then to Liverpool at the weekend")